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Why are two-stroke engines nearly unheard of in aviation?
Almost all aircraft piston engines are four-stroke engines, with one power stroke per piston every other crankshaft revolution and a great deal of intricate moving parts which are only too happy to break.  Two-stroke engines, in contrast, have a number of features that would seem to make them excellent choices for aviation use:

They have one power stroke per piston per crankshaft revolution, twice as many as a four-stroke engine, and also have far fewer moving parts, making them smaller and lighter than four-stroke engines of the same piston size and count; these two factors add up to give them a phenomenally high power-to-weight ratio, much better than that of a comparable four-stroke engine.
The same mechanical simplicity that contributes to their high power-to-weight ratio also makes them cheaper to build and sell, and (all else being equal) more reliable than four-strokes.
Most two-stroke engines are lubricated by mixing lubricant into the fuel rather than using a traditional oil sump; this allows them to operate happily in any orientation, including upside down.

The sole disadvantage of two-strokes is their greater fuel consumption and emissions, but:

Good design can reduce this problem to insignificant proportions;
Higher fuel costs could easily be offset by the reduced maintenance costs of a more reliable engine1 and the ability (due to the engine's greater power) to use fewer engines per aircraft, and;
When it comes to piston-engined-aircraft emissions, there are far worse concerns.

Yet, despite these advantages, to the best of my knowledge, only one two-stroke aircraft engine has ever entered mass-production (the highly-unusual Junkers Jumo 204/205/207 series), with a second (the Rolls-Royce Crécy) being cancelled during static-fire testing.
Why are two-strokes essentially nonexistent as aircraft engines?

1: Especially given that maintenance costs are often (though not always) a greater expense than fuel costs to begin with (scroll to page 20); the linked document is discussing jetliners, but the preponderance of maintenance costs would be much more pronounced for piston-engined aircraft, as piston engines are considerably more efficient and considerably less reliable than turbine engines (the latter due mainly to their massive mechanical complexity).

could you present some sources for "makes them ... (all else being equal) more reliable than four-strokes.", as I'm under the impression It's the other way around.

Actually there are/have been a number of mass-produced 2-stroke aircraft engines, e.g. Rotax: https://en.wikipedia.org/wiki/Rotax#Aircraft_engines

Most model piston engines are two-stroke, and the most efficient piston engines are two-stroke again, but much too heavy for aircraft use.

Don't forget about Wankel power plants: Aircraft Wankel Power Rotary Engines. They are expected to replace the horizontally opposed 4 stroke engines. Wankels can be powerful, especially when weight is not a concern. I once saw a Wankel shear a driveshaft in automotive.

The majority of powered gliders (with retractable engines, non-TMG) use two-stroke engines, specifically due to the points you list as advantages. Mind you, here the engine is mostly designed and used for full power take-off or climb, which seldomly last longer than 15min.

Actually, 2-stroke engines were fairly common in early aircraft.

@ymb1: Only the very efficient ship diesels are heavier; they use tricks like a two part piston rod to eliminate side forces on the piston. That results in a very large and heavy crankcase. The record holder's power to weight ratio is 0.042 kW per kg! For comparison: The Jumo 205 managed 1 kW per kg.

also 2 strokes engines have notoriously very poor torque output. They get their high power output because less moving parts allows  to run them at much higher speed (RPM). Unfortunately, at high rpm the efficiency suffer and the wear & tear skyrockets ...

There are many two-stroke engines in current production aircraft. They are very common when weight and volume are the limiting factors.

@PeterKämpf: Not all marine two-stroke diesels have such a massively-low PWR; the two-stroke opposed-piston diesels used in diesel-electric submarines can have PWRs of close to 1 kW/kg coupled with good reliability.

Several issues:
As you mentioned, high specific fuel consumption.  About to that of turboprops, but without the reliability. If you're going to live with that SFC, you might as well as go with turbines.
Poor reliability.  They are very sensitive to tuning and timing.  A 4 stroke piston aircraft engine will run with all kinds of things wrong with it.  Cam lobes worn down to nearly nubs.  Loose valve guides.  Horrible compression.  They can have compression readings that will cause most mechanics to ground them, but they will still make rated horsepower (Continental did testing to find this).  They are very "under-tuned", like a tractor or industrial engine.  You want this in aviation.  About the only thing that will make a 4 stroke aircraft engine blow up in your face is something like a thrown rod from a spun bearing or a cylinder that cracks open, and that requires bad maintenance or criminal neglect in the vast majority of cases.  2 strokes, in the ultralight world where they are widely used, have a reputation for letting go at any time (the only 2 stroke aircraft engine that even comes close to 4 stroke reliability is the Rotax 582 - but only close, and they're only reliable for a few hundred hours).
2 strokes require pressed together cranks with ball bearings because they have to depend on lubrication from oil mixed in with the fuel charge. So you can't have a forged one-piece crank (radials get away with ball main bearings but there is only one or two crank throws, and they still have dedicated oil lubrication).  With the only oil coming from what's diluted in the fuel in a 2 stroke, piston ring lubrication is marginal and ring life is poor.
Which gets us to longevity.  Although it has an official TBO of 2000 hours, if you take decent care of it, a Lycoming O-320 will run 3-4000 hours.  The plain bearings in a 4 stroke only see wear on startup; there is zero wear once they are pressurized because there is no metal to metal contact, and if you never shut it down and changed the oil on the go, the bearings would theoretically last forever.  Ball bearings lubricated with diluted oil have a finite life, and that and the barely lubricated rings limit longevity. You would never dream of a 2 stroke lasting that long in any galaxy in this universe.
2 strokes are popular in the ultralight world almost totally because they are cheap and have a superior power to weight ratio. You almost get a turbine's power to weight, and minimal cost, and that's worth living with the crappy reliability.  Ultralights get away with it because they crash going 30 mph not 70.
Problem is, a lot of the issues are baked into the configuration and can't be designed out.  Combine with with a business that is conservative by necessity, because your life depends on the conservatism, and it's no surprise.

Thanks for this excellent summary, it is very useful. It is possible to design two-strokes with pressurized lubrication and sealed oil sumps (the so-called "Detroit Diesel" engine works like this) but this requires a scavenging blower- and I have often wondered if it would be feasible to do this on a gasoline engine.

Yes 2 stroke diesels are another animal, and I don't know enough about them to comment.  I know that to get 4 stroke diesel aircraft engines to the threshold of feasibility, it's taken advances in metallurgy that allow cylinders that are light enough and also can tolerate the crazy multi-stage levels of boost required to achieve a power to weight equal to a normally aspirated equivalent gas piston engine.

I should also mention that the Rotax 912 series engines are 4 strokes with 2 stroke style pressed cranks and ball bearings and a normal pressure lubrication system.  They are quite reliable and I believe the pressed crank works because they spin through a gearbox at high rpm with a damping coupler so the crank doesn't have to tolerate the torque pulses a low rpm direct drive engine does.

@JohnK But wouldn't a 2-stroke diesel mean half the cylinder size for the same output as a 4-stroke diesel, mitigating the weight issue?

Am I right in thinking that a simple 2-stroke loses all lubrication at zero throttle? The propeller would be windmilling and driving the engine with no lube. This isn't a problem in lawnmowers which stop when fuel is cut off but in cars such as the Trabant a separate lubrication system was required to allow downhill coasting.

It's idling with idle fuel coming in, so it's getting a little, enough to do the job with the minimal loads on rings and bearings.  Ball bearings need very little oil to function since the only sliding actions are the balls rotating in the cage. The main disadvantage of them is they do have rolling metal to metal contact and gradual wear is inevitable.

@John K: But most modern automotive 4-stroke engines have deceleration fuel cutoff, so that when coasting at zero throttle they use no fuel.

@jamesqf Not sure how that could be done with a 2 stroke because if it's windmilling the parts are reciprocating with no lube.  In any case you won't see that kind of thing in aviation because you are effectively shutting down the engine any time you reduce power to idle, and this creates problems even with 4 strokes.  You may be at idle for extended periods and then you're left with a dead cold engine.  You could do that in theory with any normal aircraft engine if you wanted, because they all have an idle cutoff mixture setting you set manually, but nobody in their right mind would do it.

@nielsnielsen 'the so-called "Detroit Diesel" engine' why "so called"?  "Detroit Diesel Corporation" is a company specializing in the production of diesel engines, particularly for the heavy duty truck market in North America.  They are now part of Mercedes-Benz.  They used to have a very-popular two-stroke engine, but it was impossible to keep up with emissions regulations with a two-stroke.  (Source, I used to work for a company which developed engine controllers for them.)

@MartinBonner, I say so-called because not all blower-scavenged two-stroke diesels are made by them, even though such engines are commonly called detroit diesels no matter who made them. Before being bought by MBZ they had become part of the Allison enterprise and were called Detroit Diesel Allison, but that did not cause people to call a blower-scavenged two stroke diesel a detroit diesen allison engine.

@JohnK All of what you said is on top of the cost of 2-stroke oil - the IAME 2 stroke engines we run on karts need about 1 oz of oil for every 8 of gas. Some oil will cost you about $1/oz - which is orders of magnitude more expensive than gasoline. That, then, in aviation proportions, would likely make the cost of running a 2-stroke aircraft astronomical (and enough to make it financially impossible, by itself).

@John K: But you can get the same sort of (nearly) dead cold engine in a modern car, when descending ~4500 ft of mountain road in the winter - something I do quite regularly.  With computerized fuel injection, it doesn't really seem to be a problem at all - except when you'd really like a little heat for the defroster and such.

@nielsnielsen Ah!  I hadn't realized it was a generic term as well.  Thank you.

The main reason there are few 2 stroke aircraft engines is likely the reason there few gasoline 2 stroke engines above 125cc in general: They have poor fuel efficiency and poor emissions. You note that aircraft engines still use leaded fuel and suggest this is a greater concern, but the concerns mentioned above exacerbate lead pollution, due to the greater fuel requirements and emisions of unburned fuel. You say this can be solved with good design, but that comes with cost and complexity. After a certain amount of time trying to design a 2 stroke cylinder that won't bypass fuel/air mixture from the inlet port to the exhaust port, you give up and design a 4 stroke instead. It'll weigh more but the decreased fuel usage offsets the weight penalty for anything that flies greater distance than an ultralight.
Diesel 2 stroke engines generally have too poor a power/weight ratio for aviation, but very sucessful in other fields. The fuel is injected only when the piston is in a raised position, so the engine ingests only fresh air in its intake manifold, and if some bypasses to the exhaust it doesn't waste any fuel.
https://en.wikipedia.org/wiki/Category:Aircraft_piston_engines lists 79 two stroke engines, more than half the number of radial engines at 146, so obviously significant, and for some reason the Junkers engines you mention are listed as diesel but not 2 stroke.

There is nothing inherently inferior in two-stroke engines that would prohibit them from being used in cars or aircraft.
As stated by OP, 2-strokes have many properties that should be of value in aircraft, particularly the higher power-to-weight ratio of generating a power stroke at every revolution. They can also be designed for simplicity and low moving parts count, like a lawn mower: no valves but ports, pre-mix fuel and oil without a separate oil pump, and resulting in an engine with poor specific fuel consumption and high pollution count. But of course for modern day cars and aircraft, that design won't be used. For instance from this article (2nd answer):

In a two stroke the transfer ports and exhaust ports are open at the same time. At some engine speeds, portions of the fuel mix go straight from the transfer ports out the exhaust without being burned. This leads to high fuel consumption and the ejection of unburned hydrocarbons. A solution is to use direct injection in which fuel is injected into the cylinder only after the ports close. This drastically reduces both problems.
The other source of pollution comes from the total loss lubrication in which unburned two-stroke oil passes directly out the exhaust ports. A solution is to replace the total oil loss system with full crankcase lubrication as found in four-stroke engines. To do this, the crankcase can no longer be used for scavenging, therefore a supercharger needs to be used for supplying the scavenging air.

So a two-stroke engine not designed for simplicity and low cost, will have no unburnt fuel and no total loss oil exhaust. It will still have double the power strokes of a comparable four-stroke. And it will still have a smaller parts count than a four-stroke with an oil sump and a supercharger. Some examples:

Image above is from an article in the NYTimes, on a modern design of a 2-stroke automobile engine. With valves instead of ports. With a compression ratio that can be varied: lower compression for petrol due to knocking prevention, high compression for diesel for higher Carnot cycle efficiency.
The Junkers Jumo 205 mentioned in OP was a 2-stroke diesel with intake and exhaust ports, with a clever arrangement allowing for better timing of the fuel/exhaust cycle. Designed and manufactured in the 1930s(!). Then abandoned, according to this article because of the two crankshafts connected to one propeller creating complexity, plus it had to be installed vertically to accommodate the oil scavenging system.
Marine 2-stroke diesels, with injectors, valves, and a twin piston layout, as used and operated for decades.

So why are two-strokes not used for aircraft propulsion anymore?

The aviation industry is super conservative and new developments take a long time and investment in order to be accepted in the mainstream. As illustrated by the Thielert insolvency - if applying mature diesel engine technology to GA aircraft results in bankruptcy, where would that leave companies developing a new 2-stroke technology?. A newly developed 2-stroke piston engine would be used for smaller aircraft, which simply cannot generate the revenues and numbers for significant Return On Investment. Development of car/GA aircraft size petrol 2-stroke was simply stopped many decades ago, while 4-stroke continued being refined.
There is an inherent limitation in two-stroke technology: there is much less time to expel the exhaust gas and draw in fuel, because each stroke is a power stroke. This limits either the cylinder volume, or the max. RPM. Not a problem for diesels, ship diesels run at a very leisurely 150 RPM. Four-stroke petrol does not have this limitation. Diesel can run at super high compression ratios generating an enormous amount of torque per stroke, petrol cannot. So petrol 2-strokes run into a power limitation per cylinder. Which can also be solved with new technology - another Thielert saga?

The issues mentioned by the OP and other responders are peculiar to a few specific engines that are not comparable to each other and mostly don't pertain to prop drive applications. Nobody would design a 2 stroke replacement of a Lycoming as described. It would look very similar and be configured very similar to the 4 stroke. The issue is specific power and specific fuel consumption of a prop drive at a given capital expense and operating expense. Power density is not much of a driver, and can be much lower than that desired in the automotive sector, for instance.
A 2-stroke would be a bit more complicated, in that it would need a blower to manage air charge. It would have essentially the same oil system and fuel system as the 4 stroke. It might use poppet valves in the head for intake and exhaust, or it might use ports. There are at least 10 basic variations on how to get air in, light it off, and get the exhaust out of a 2-stroke. But only three or four are worth serious consideration here. And there probably isn't enough difference among those to worry about as far as this question is concerned, so lets just assume a roots blower feeding charge air to a poppet valve headed engine. Maybe it's direct injection or maybe indirect injection.
Specific fuel consumption favors the 2 stroke over a large range of sizes and speeds, but probably not at this small a scale. It's probably a wash at the size of a Lycoming. The savings in internal friction exceeds the blower losses for larger cylinder sizes. But for a light aircraft, the blower wouldn't be the all that great. Gensets operating at comparable rpms still favor 4-strokes.
One of the biggest factors effecting weight is compression ratio. Since we can trade power density for specific power, we go big and low CR and thin walled. This gets the weight down, especially when you can't run high rpms to begin with. This may be where the 2 stroke loses it's advantage. It's good when you need powerful, light and small, but if you don't need the small part, There isn't much of an advantage. It will be a bit smaller, but not necessarily any lighter when built to drive the same prop.
The sort of cylinder design and engine architecture that you want is almost entirely determined by the application, not by whether you are running a 2-stroke or 4-stroke. For the most part, you can take any 4-stroke and convert it to a 2-stroke without all that much trouble. A buddy of mine started converting junked Cadillac 500 cid motors to 2 strokes when he was 14 years old. He bought a house and built a machine shop when he was 16.
So I would say that there isn't any real reason to not develop a 2-stroke for small aircraft, but that none of your preconceived notions about the differences between the two types pertain to this application. I think the dynamics of the intake air might be harder to manage (read more expensive and maintenance intensive) in the 2-stroke compared with the 4-stroke. The requirement for reasonably fixed power at reasonably steady rpm given widely varying charge conditions is a very different problem than that facing the automotive sector, which has steady charge conditions and widely varying power and rpm requirements.

Many UAVs use two stroke engines due to their low weight and high power output. The resulting high RPMs are matched with relatively small diameter propellers which is fine for the power, speed and size constraints involved. Examples include the Boeing / Insitu Scan Eagle and RQ-21A Blackjack / Integrator. Their specific fuel consumption is 33% to 50% worse than a well-tuned automotive four stroke engine but similar to the smaller four stroke engines that would otherwise be used.

Because 1-cylinder engines aren't powerful enough
2-stroke engines have a thing happen with the cylinder at bottom dead center.  Exhaust gas is pushed out of the cylinder by intake air, which is actively pumped into the cylinder. This is called scavenging.
2-stroke engines, as you imagine them, use the crankcase for an air pump. The piston going down compresses air in the crankcase, and when the intake port is exposed, scavenging occurs.
Guess how many cylinders you can have in a 2-stroke engine. ONE.
Becuase if you have 2 cylinders, then the air volume of the crankcase does not change, and it's not usable as a pump.  (Well if it's exactly two, you can have a 180 degree crank so they both top out at once, but that pretty much defeats all the advantages of two cylinders.)
2+ cylinders require blowing
With 2 or more cylinders, the crankcase is useless as a pump because crankcase volume does not change.  (On the upside, you can use a normal oiling system, and you no longer need to mix 2-cycle oil; of course now more complexity has crept in.)
You must use an air pump of some kind to accomplish scavenging.  So now you've added a bunch of complexity.  And worse, the blower will perform quite poorly at altitude.
"Oh, we know the fix!  We'll just use a turbocharger".  Great kid, but how are you going to start it? The engine needs to be blown at all speeds, even during cranking - so a turbo is useless at low power settings.  You need an overrunning clutch to the engine can mechanically drive the turbocharger at low speeds/start. More complexity.
Further, if the intake and exhaust are both cylinder-side ports, it creates some serious issues with ring wear and carbonization.  Further, the funny shape of the piston (to get air to flow the right way) greatly complexifies fuel burn, and limits maximum compression.  So real soon, as your 2-cycle engine gets bigger, you need to move intake or exhaust to valves, and because of rings and carbonization, exhaust valves make more sense than intake.  So now you're right back where you started with valves in the cylinder head.
However, intake ports have their own problems: the fuel/air mix will come in direct contact with the piston rings, and unevaporated fuel will tend to wash the oil in the rings, increasing ring wear and diluting the oil.  This is not a problem for a diesel, since they direct inject into the cylinder.
So congratulations.  You have an engine powerful enough to sub for a Continental or Lycoming, and you have

An oil pump
Cams and exhaust valves
A blower
Direct injection to solve the ring-wash problem

So that's a simplicity fail.

Not true: back in the day of 2-stroke outboard motors, Mercury made a 6-cylinder in-line 2 stroke rated at 120HP and both Evinrude and Johnson made 75HP V-4 two strokes. The original Saab 93 car had a 3-cylinder 2-stroke in it. In each case, the crank throw for each cylinder was sealed off from all the others so the bottom end could be used as an air pump. There were also a large number of motorcycle engines that were 2- and 3-cylinder 2 strokes that featured isolated crank throws as well. The McCulloch  flat 4-cylinder drone engine was also a 2 stroke engine.

Absolutely untrue, -1. For instance check out the wiki page for the Yamaha RD350, a twin cylinder 2-stroke motorcycle from the 70s.


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Why don’t flying-boom tankers have collapsible/frangible/breakaway refuelling booms?
There are two main types of aerial-refuelling systems in use today:

Probe-and-drogue systems reel out a hose behind the tanker, terminating with a soft conical canvas bucket; the pilot of the recipient aircraft manoeuvres their aircraft so as to insert a forward-mounted refuelling probe into the tip of the bucket, completing the hookup and allowing fuel to flow down the hose and into the recipient aircraft.
Flying-boom systems use a rigid boom mounted on the underside of the tanker; a dedicated boom operator on board the tanker (traditionally seated at their own isolated station next to the boom’s shoulder joint, but nowadays in the cockpit with the rest of the crew, controlling the boom via remote control and viewing it via CCTV) flies the tip of the boom into a receptacle on the recipient aircraft using flight control surfaces mounted on the boom’s wrist, whereupon the boom engages a valve inside the receptacle, completing the hookup and allowing fuel to flow down the hose and into the recipient aircraft.

In either case, if the recipient pilot isn’t careful (for flying-boom tankers, the boom operator is another potential culprit), the recipient aircraft can collide with the refuelling equipment; with a probe-and-drogue tanker, this is a (relatively!) benign occurrence, since the soft canvas drogue, attached to its long, flexible hose, can usually just bounce harmlessly off the aircraft (although damage, occasionally severe, can still occur to the recipient aircraft on rare occasions).
However, with a flying-boom tanker, the hard, rigid structure of the refuelling boom (coupled with the fact that, unlike with the probe-and-drogue system, both the recipient and the boom are actively manoeuvring [the recipient to match the motions of the tanker and correct for the turbulence created by said tanker, the boom to engage with the fuelling receptacle], considerably increasing the potential closing speed between the two should the recipient correct back towards the tanker at the same time as the boom operator adjusts the boom’s trajectory back towards the recipient) makes things considerably worse.  As the boom is solidly-built and not designed to break away upon impact, the impact forces generated by a collision between the recipient and the boom are transmitted in full force to a fairly small area of each aircraft (the boom’s shoulder joint in the tanker’s case, and wherever the tip of the boom happens to hit on the recipient), which will likely destroy the boom and cause (at the very least) severe structural damage to the tanker’s lower aft fuselage, and, on the other end, can (depending on the point of impact) easily be enough to (say) tear through even a large, heavy, stoutly-built aircraft’s upper fuselage and then snap one of its wings right off the fuselage; should any fire or explosion resulting from the impact travel up the boom, the tanker is probably doomed as well.
Most things with a fairly high risk of being hit by aircraft (approach lights and ILS antennae, for instance) are made to be frangible (harmlessly disintegrating upon impact), to minimise the potential damage caused to an aircraft that happens to hit them; likewise, most things that stick out the bottom of an aircraft (such as engines and landing gear) are made to break away cleanly if excessive force is applied in the wrong direction, rather than risking damage to things like fuel tanks.  The same principles could conceivably be applied to refuelling booms, which, like approach lights or ILS antennae, can easily be struck by an aircraft if one of the pilots involved does something even slightly wrong (and which one would logically expect to actually be much more frequent victims of errant aircraft than approach lights or ILS antennae, given the relative extreme smallness of the margins of error involved), and which, like engines or landing gear, could potentially cause severe damage to their attachment areas by transmitting excessive impact forces (and which one would logically expect to be much more likely to be subjected to excessive impact forces than engines or landing gear, given that landing accidents are rare, whereas aerial refuelling is literally an everyday occurrence), such that, if a recipient aircraft came in a bit too fast, or a boom operator swung the boom a bit too hard, and a boom-recipient collision resulted, the boom itself would give way before either the tanker or the receiver suffered severe structural damage.
Designing a boom to break away in a collision would, admittedly, make the detached boom a piece of potentially-hazardous flying debris, which could strike the recipient aircraft, or be sucked into an engine, and, thereby, cause additional damage; however, this would very likely be preferable to the major structural damage, or complete destruction, incurred by a recipient aircraft involved in a significant collision with a non-breakaway boom.  Alternatively, the boom could be made collapsible, such that, if subjected to excessive axial compression loads from a collision, it would telescope into itself, or possibly be pushed into the tanker’s aft fuselage (this would require that space be provided for the boom to collapse into the fuselage without damaging anything; it should not be particularly hard to find space for this, however, given that even tanker aircraft have to leave large empty spaces within the fuselage in order to be light enough to take off), rather than standing firm and causing severe damage to one or both aircraft.
So why are frangible/breakaway/collapsible refuelling booms not (to the best of my knowledge) a thing?

The booms DO telescope, both under the boom operator's control, and also in response the relative motion of the tanker and receiver. Too far apart, the nozzle disconnects; too close, the boom operator calls for a breakaway.  The hypothetical hazards posited in the question are quite rare, in practice.

The additional complexity probably neutralize all the benefit.

Another problem that must be addressed with easily breakable booms is the (ignitable) fuel vapors released at breakup.

"given that landing accidents are rare, whereas aerial refuelling is literally an everyday occurrence" Comparing apples (landing accidents) to oranges (aerial refuelling). Landings occur literally daily, heck there's probably a plane landing every minute of every day somewhere in the world. How frequently do aerial refuelling accidents happen?

You answered it yourself: the potential for damage from a frangible boom is far more than one designed to stay together. On a boom system the refueling probe is rigid, but impact velocities tend to be slow as airplanes in formation are moving very slowly compared to each other. Hitting the probe may bend something, but rarely causes catastrophic damage. The most serious damage comes when the airplanes themselves collide, having a frangible boom won't mitigate that.
Consider a boom designed to break off: as soon as it is free the slipstream accelerates it, and by the time it hits the tail it's moving pretty quickly - fast enough to cause serious damage. The risk from a break-away boom is far higher.

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Why do pilot and GPWS approach callouts say “minimums” instead of “minima”?
A safe instrument approach requires a number of callouts along the way; two of the most important come fairly late in the approach, first to alert the pilot flying that the aircraft is approaching the minimum descent altitude (MDA) for that approach, and then when the aircraft actually reaches the MDA (these callouts were traditionally made by the pilot monitoring, but are now usually made by the aircraft’s GPWS).  At this point, if the pilots have the runway environment clearly and consistently in sight, they can continue and land; otherwise, an immediate missed approach is mandatory.
These callouts are typically annunciated in plural form, as, respectively, “approaching minimums” and “minimums”; however, the plural of “minimum” is not “minimums”, but, rather, “minima”, which is also shorter and takes less time to say than “minimums” (and good time management is one of the many many things that are critical during an instrument approach, especially one in low visibility or over or near obstructed terrain), so why do these pilot and GPWS callouts typically use the longer and ungrammatical “minimums”?

Is there a good reason to require non-native speakers to learn the arcane rules for pluralizing certain words when the obvious form is also acceptable?

I'm convinced the voice of the Rockwell Collins EGPWS callouts is that of Brent Spiner, the character Data in Star Trek NG https://en.wikipedia.org/wiki/Data_(Star_Trek).  Every time I'd hear it I'd think "yeah that's Data".

Note that cockpit is a noisy environment and you can get a radio transmission with its notoriously terrible audio quality over the announcement. In such environment, longer words have advantage of being harder to miss or mistake for another. Compare e.g. use of ‘negative’ instead of ‘no’.

We say many things in cockpits that aren't normal usage, the answer as to why is usually because it works.

By the way: "an immediate missed approach is mandatory" this is not true when reaching the MDA on a non-precision approach. You are allowed to maintain the MDA until reaching the MAP (Missed Approach Point).

@JanHudec the EGPWS callouts come over the internal audio system and have FM broadcast clarity.  Even the VHF comms with the newest equipment are starting to approach FM audio clarity (if not frequency response).  I did a ferry trip from South America to North America once, my first and only time down there, and the ATC audio instruction in SA was more or less: "bwabwa bloo bloo maytay wenny wousn fee" and we'd look at each other and go "what the hell did he say???".  Entering US airspace from the Caribbean, contacting the first US center controller, it did almost seem like FM radio.

@JohnK, the callouts themselves have good quality, but you can still get radio transmission (of any quality) and the callout at the same time.

But using "Contact" as a verb is just... wrong - you whippersnappers!

Minimums is an acceptable plural of Minimum (along with minima).
As for why its used as a call out is likely because it is the preferred pluralization used by the FAA and in my experience the term generally used when talking about "approach minimums" or "weather minimums".
Its worth noting that the FAA uses both minima and minimums in their handbook.

Interestingly, the Oxford English Dictionary says that minima is the British plural and minimums is the US one.

My experience as an American says that very few of us use "minima", while my experience with members of the British Commonwealth indicates that "minima" is much more common. (Most of that BC experience is via TV, so take that with a grain of salt.)

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Why was the 307’s pressurisation system removed when it was modified into the C-75, despite the fuel-burn advantage of higher-altitude flight?
The Boeing 307, an airliner derivative of the B-17 heavy bomber, was the first pressurised airliner to enter series production and go into service (hence its bulbous, altogether un-B-17-like fuselage, the better to hold pressure), allowing it to cruise at higher altitudes,1 where the air is thinner and less draggy, without incapacitating its occupants.  As World War II broke out shortly after the 307 was introduced, only ten ended up being produced (out of which only nine still existed, the first off the line having already been destroyed in a crash), and eight of the nine surviving 307s were drafted into military service;2 Pan Am’s fleet of three flew military contract flights in Latin America and the Caribbean, while TWA’s five 307s were extensively modified for transatlantic VIP and freight carriage (being redesignated as the C-75).
One of the modifications applied to the ex-TWA aircraft3 was the removal of their pressurisation system, ostensibly to save weight; this restricted the C-75s to an altitude safe for unpressurised human beans (approximately 8 kilofeet or so max, as opposed to the 12 kft of the pressurised 307), with resultant drag and fuel-burn penalties in the denser, lower-altitude air (one of the other modifications was the installation of numerous additional fuel tanks to sustain the aircraft across the Atlantic).
This does not seem to make sense, given that the main advantage of cabin pressurisation is the aforementioned ability to fly at higher, less-draggy altitudes, where an aircraft cruises at a (sometimes much) higher true airspeed and burns less fuel, allowing it to carry more paying payload and not as much fuel.4  Thus, even a relatively-low-ceilinged aircraft like the 307 should still have been able to carry more payload, or carry the same amount of payload farther, when pressurised than when unpressurised, even factoring in the extra weight of the pressurisation system, due to the lesser fuel requirements of (and, thus, lighter fuel load needed for) flight at pressurisation-enabled higher altitudes - so why did the USAAF remove the 307s’ pressurisation equipment?

1: Albeit not that much higher, due to its low fuselage pressure rating and its lack of a passenger emergency oxygen system.
2: The remaining 307 (the second one off the production line) was Howard Hughes's personal airplane (he hardly ever used it, though).
3: Which would be returned to TWA at the end of the war, and converted back to commercial passenger configuration (with a number of improvements gleaned from wartime B-17 work).
4: Contrary to what those less-well-informed about aviation tend to believe, the purpose of cabin pressurisation is not to allow airliners to fly above bad weather; even modern jetliners, which can cruise at up to FL410 (~41 kft), don’t fly high enough for that.  You’d have to go up to at least about FL550-FL600 (~55-60 kft) to overtop storm clouds, something of which only one airliner ever produced was capable (and said airliner has now been out of service for over a decade and a half).

Note 4 is absolutely NOT true. Weather avoidance was a huge benefit to pressurization, along with efficiency. Storm clouds extend only a few thousand feet above the tropopause into the stratoshpere (the anvil).  The tropopause is around 50k only near the equator and up to maybe 30 deg N/S.  In the mid latitudes, it's in the 30s and you can easily top thunderstorm anvils when cruising above FL350.  Even if you can only get to 25000, you can still avoid 90% of nasty weather, which, apart from thunderstorm cells, pretty much means icing.

"Kilofeet"?  Never heard that one before.

Pressurization is primarily for crew and passenger comfort. The B307 was a very early attempt at cabin pressurization and the marginal pressurization differential of only 2.4 psi did not allow much of an increase in cruising altitude.
The difference between 8,000’ and 12,000’ did not really justify the added expense and weight.
When the B307s were converted to C-75 military cargo configuration, there was even less justification for pressurization as military crew and passengers would not expect the comfort of pressurization. The C-75 could still fly at altitudes up to 23,000’ but the crew and passengers would just need to use supplemental oxygen.
The B307 had Wright R-1820 engines which were similar to B-17 engines except they were only supercharged and lacked turbochargers. Cabin pressurization was supplied by compressors mechanically driven by the engine. These compressors would rob the engines of power so removing them would allow the aircraft to fly higher but with less comfort.

Actually, it couldn't fly much above 12 kft, even unpressurised, since going higher than that robbed the engines of much of their power.

It could fly much higher than 12,000’ but with reduced power it would just be slower and even more economical. I suspect the Capt Adams quote only applied to passenger airline operations where speed was a major consideration.

It had the B-17's engines which had the same General Electric turbocharger system the P-38 had.  The B-17's service ceiling was 35000 ft, although it was normally limited to under 25000 for accuracy and to avoid giving crew members the bends.

The Boeing 307 had Wright R-1820 engines which were similar to B-17 engines except they were only supercharged and lacked turbochargers. Cabin pressurization was supplied by compressors mechanically driven by the engine.


text:
Do I need a trim tab system on my twin tail?
I plan on building a single engine H-tail aircraft with retractable landing gear, removable wings for transportation, and either split flaps or plain. My only problem is, do I need a trim tab system to handle the workload, or should the air from the prop not interfere with the lift and cause problems with aileron, elevator, and rudder control, increasing the muscle the pilot must use to handle the aircraft?

You need to be able to trim the aircraft for different loading/speeds. It doesn't change the "muscle required" to handle the aircraft, just how much muscle you need to hold the aircraft in a specific attitude.

Yes a trim system would be a most excellent idea. If you can trim the aircraft such that it just flies straight on when you release your hands and feet off of the flying controls, the flight will be more enjoyable than when you need to keep pulling one way for the duration of the flight, like walking while holding up a briefcase in front of you.
Trim adjusts the neutral point of the flying controls: the point where the stick returns to when released.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Which flap is better suited? Split or Plain?
I’m considering which flap to build onto a home built aircraft, but not exactly sure which flap would best fit the aircraft and which one would be more beneficial. The aircraft has a fighterlike look to it, but I’m mostly worried about how the flaps will effect airflow during landing and takeoff.

We probably need a little more basis than "a fighterlike look to it" to give anything approaching meaningful advice. Split flaps were mostly used on fighters with very high power to weight ratios, so the high drag of the flap wasn't a major problem. If you don't have that kind of power, you might want to consider slotted flaps instead (they also increase lift, but generally don't increase drag nearly as much as plain or split flaps).

Me personally, I would try to approach building an aircraft with some engineering-like methodology (being an engineer sort of biases me here I guess). That would include calculations on expected mission and flying performance (do I need flaps, and how much additional lift do I need/drag can I afford) as well as thoughts on structural integration and construction. Such reasoning would help influence my design decisions like which type of flap to use. There’s many good books on aircraft design. Why not start there?

It's great that you're building an aircraft, but it's a huge task and especially if you want to design it yourself rather than buy a kit or plans. In other words, you're going to have questions that require detailed discussions and this site isn't a good format for those. If you haven't already done so, I strongly suggest you join the EAA (or local equivalent) and connect with other builders who can be a source of advice and help. You're always very welcome to ask questions here, but keep in mind the limitations of the Q&A format.

As usual, the answer is: It depends.
A short comparison would read like this:

Plain flaps have better L/D when deployed at small deflection angles.
Split flaps have a higher maximum lift at the same deflection angle.

Therefore, a plain flap can be used for take-off or slow flight, as well. Split flaps are only good for landing, but in this they are better than plain flaps because the increased drag makes the landing easier.
Airflow on the bottom of the airfoil is very similar, and on the top the plain flap will exhibit flow separation somewhere along the flap chord while the split flap has a well defined separation point at the trailing edge. This should produce a more steady wake and more linear characteristics of the split flap when compared to a plain flap.
Proof: Figure 7 from Chapter 5 of Sighard Hoerner's "Fluid Dynamic Lift":

I’m glad people are willing to share their knowledge with others. This acquired understanding could help amateur pilots/ home builders to build and fly safer planes but also to enjoy the experience. I’m currently studying the FAA-H-8083-3B Airplane Flying Handbook to learn the roots. I’ve also constructed a 3-D model of aircraft control systems to benefit in a better comprehension of how things work, which way the parts move, and what to look for during construction.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What do compressor/turbine stages mean, and why we need a different number of stages?
I am new to aviation. I realize that LP (Low Pressure) and HP (High Pressure) compressors/turbines have a various number of stages.

What do stages mean?
What is the reason to have a different number of stages for LP and HP?

For question 2: Why do large turbofans generally have many more LP turbine stages than HP turbine stages?

A piston engine compresses air by enclosing it in a chamber and then "closing in the walls", so to speak, with the piston so that the air has nowhere to go and it gets mechanically compressed.  The piston engine is a movie theater with the doors locked and walls that close in.
On a jet engine, you are compressing air by squeezing it into a smaller space in a constant stream, depending on the velocity and inertia of air mass to do the squeezing.  It's more like a movie theater where there is an open exit and you yell "Fire" so people at the exit are being squeezed together and crushed by the mass and inertia of the larger crowd behind them all being forced into the same spot.
For this to work efficiently and get a decent compression ratio, you have to impart energy to the air stream all along its path down the converging passage it's on.  It's like having the theater exit being a converging hallway with people stationed along its length, in stages you might say, giving theater goers a big shove so that by the time they are going through the small exit they are running at full speed and everybody is able to make it through.
An axial compressor is just a big fan with multiple blade elements, stages, (with stators in between to manage the flow direction for the next stage) that moves the air along in baby steps all along the convergent path to the inlet of the burner can.  How many stages is a design balancing act - more stages, more compression, but more weight and complexity; so somewhere on a given engine there is a sweet spot in the number and design of stages and the designer has to find it.
Engines get broken up into LP and HP compressor/turbine sections when it's desirable to split the compressor into independent segments, each with its own turbine to drive it, that are free to turn at different speeds, which makes the engine able to handle larger variations in inlet flow.  The number of stages in each section is a function of the compression ratio the designer wants to achieve with each compressor section.  A dual spool configuration like that adds weight and complexity, so there has to be a payoff operationally to be worth it, so you will some engines with two spools and some without.


text:
How does TCAS work?
TCAS is obviously a fantastic piece of kit, but I was wondering: exactly how does it work?
What I mean is, how does it determine whether to tell a pilot to climb or descend when there's traffic?
Also, can you give an overview of the various types of TCAS, and how planes agree a resolution?

TCAS works by sending interrogations to other aircraft's transponders. The transponder will reply to the interrogation in a similar way it responds to radar. From the time difference between the interrogation and the reply, the distance to the other aircraft is calculated. The reply itself contains the altitude of the other aircraft.
The explanation below gives an insight in the basic working of the TCAS algorithm. In reality it is more complex to deal with all kinds of exceptional situations.
The distance and the altitude difference with the other aircraft is tracked to identify a trend. From successive distance measurements the closure rate is determined. With the closure rate and the current distance (slant range) an approximation is made of the time to the closest point of approach (CPA). This is done by simply dividing the range by the closure rate, the result is called 'range tau'. The same is done in the vertical plane. Dividing the difference in altitude by the vertical speed difference leads to the vertical tau.
If both tau's are less than certain threshold, a Traffic Alert (TA) is raised. When the tau's are less that another (lower) threshold, a Resolution Advisory is given. The TA is a 'heads up' indication, the RA is an instruction that must be followed by the pilot to reduce the collision risk.
The threshold times depend on the altitude, ranging from 20 seconds (<1000 ft AGL) to 48 seconds (> FL200) for TA's and from 15 seconds (<2350 ft) to 35 seconds (> FL200) for RA's. Below 1000 ft AGL, RA's are inhibited.
Originally TCAS did only give Traffic Advisories in the form of a "Traffic, traffic" annunciation. There was no avoiding action indicated.
With the introduction of TCAS II in the second half of the '80s the Resolution Advisory made its entrance.
RA selection
In selecting a resolution advisory, there are two basically two steps. The first step is to select a sense, either upward or downward. This is based on a calculation of how much altitude difference can be achieved at the CPA by either starting a climb or descent, assuming the target will maintain its vertical rate. In the calculation the reaction time of the crew and acceleration and climb / descent rate of the aircraft is assumed (5 seconds, 0.25g, 1500 fpm).
The second step is to select a magnitude. The algorithm is designed to be the least disruptive to the flight path, whilst still achieving a minimum vertical separation.
RA coordination
When a RA is selected, it is transmitted to the other aircraft. When the other aircraft receives that message, it will only use the opposite sense for its own RA. In the rare case that both aircraft transmit their RA intent at the same time, the aircraft with the higher Mode S address will give in and reverse its RA sense if it is conflicting with the other.
Algorithm versions
Over the years various improvements are made to TCAS II. The initial TCAS II systems had version 6.0 of the algorithm, later replaced by 6.04a in the mid 90's. Further system evaluation lead to the development of version 7.0, implemented from 1999. Following a near mid-air collision in Japan in 2001 and a mid-air collision over Uberlingen, Germany further changes were made leading to version 7.1.
Changes that were made include:

the wording of the RA's
logic to limit the vertical path deviations
logic to prevent nuisance alerts in RVSM airspace and on closely spaced parallel runway operations
logic to detect non-compliance to an RA
improved RA sense reversal logic

TCAS & ATC
TCAS RA's take precedence of ATC instructions. The fact that one of the two aircraft followed ATC instructions, and the other TCAS RA's contributed to the Uberlingen accident.
In case a pilot is getting a TCAS RA he should notify ATC.
==========
The FAA has produced an Intro into TCAS II v7.1 which provided some interesting reading on the topic.

The RA is not negotiated. The two system carry out the same calculation from the data they both know, which basically means just the altitude of both aircraft which are broadcast. Remember, it needs to work with mode C transponders that are rather dumb.

@JanHudec As of TCAS II, they do indeed negotiate a solution (if they are both properly equipped) that prevents both aircraft from coming up with the same solution and making the problem worse.

Thanks for your answer! I was hoping for a more technical answer on exactly how an RA is negotiated, and the differences between various TCAS versions (like @Lnafziger pointed out).

@DannyBeckett simply put climb is called when this plane is higher than the other plane and vice versa. Also I believe the TCAS RA takes priority over traffic control (as part of See-And-Avoid)

@ratchetfreak When responding to an RA that has priority over ATC (you are correct) and you'll make a call "Approach, N1234 TCAS Climb" to inform them you are complying with an RA.

@ratchetfreak It is not simply which aircraft is higher. The algorithm will calculate if that would work, and if it does it is certainly the preferred solution. But if the aircraft above is descending, it might be instructed to descend faster if reversing to a climb would take too long.

Fantastic edit! Very interesting, thanks.

Suggest reading the Intro to TCAS II V7.1 from the FAA.

The TCAS standard uses the standard transponder, having the interrogator on board in addition to the transponder itself.
The aircraft periodically broadcasts the interrogation signal. The power is less than from the ground radar, but sufficient to trigger response from aircraft that may be less than a minute of flight.
The distance to the other aircraft is determined from the timing the signal, closure rate from two consecutive measurements of distance and pressure altitude is part of the transponder response. Position can only be determined approximately by using directional antenna, so it is only used for showing the other aircraft on the nav display, but not for determining the resolution.
The system activates if the closure rate is such that collision could possibly happen in some specific time. That does not mean that the aircrafts are actually on colliding courses; the TCAS system does not evaluate headings or positions. It is just that the momentary closure rate is high enough.
If the aircraft could collide in 40 seconds, the system gives out a "Traffic Advisory". This is just a warning that there is a nearby traffic and that an action may need to be taken soon.
If the distance reduces so that the risk of collision gets to 25 seconds, a "Resolution Advisory" will be given. This is an instruction to climb, descend, climb/descend faster or level out. The pilot is required to comply in 6 seconds, the instruction has priority over ATC instructions.
Which advisory to give is derived by a set of rules from altitude and rate of climb of this and the other aircraft. Since the altitude is sent in the transponder response and rate of climb is simply difference of consecutive altitude measurements, both TCAS have the same information and therefore will always come to the same conclusion. Basically it will tell whichever aircraft is higher to climb, climb faster, or if descending to not descend and whichever is lower to descend, descend faster or if climbing to not climb.
There is also similar system for light aircraft called FLARM. The principle is similar. It uses lower power radios, because gliders and ultra-lights don't have enough power for the standard transponder. It's messages also include position and heading, so it can also give lateral (turn left/right) instructions. It is however incompatible with TCAS.

TCAS has nothing to do with the flight rules, it works regardless of IFR/VFR

TCAS I: only needs Mode-A transponders, only provides "Traffic Traffic".
TCAS II: needs Mode-C transponders, provides "Climb Climb" etc.
TCAS III: needs bigger computers, provides "Climb Left" etc.
TCAS III has never been produced - TCAS II is good enough to do the job, and nobody's worked out a good algorithm for it yet!

Please consider providing citations for your statements.

TCAS I requires Mode C (TSO C118, RTCA DO-197).  TCAS II requires Mode S (TSO C119, RTCA DO-185).

honestly, I'm wondering  about which aircraft who climb or Descends first


text:
What's a TRA and how does it differ from a TFR?
This is from a recent email from the FAA Safety Team:

The FAA will soon begin limited use of a Temporary Restricted Area
  (TRA) in certain areas when the types of operation(s) to be conducted
  there require a TRA. The flight rules that apply to a TRA are the same
  as a Restricted Area (RA). The best FAA resources to view any TRAs
  upcoming or active are the FAA Special Use Airspace (SUA) website and
  the Notices to Airmen Publication (NTAP).

Usually when the FAA wants to temporarily restrict access to airspace it puts up a TFR and it isn't clear from the wording of the email what a TRA is, why the FAA would create one rather than a TFR, and what the operational differences are between a TRA and TFR.
What is a TRA and how does it differ from a TFR?

I think that they are referring to an extension of the restricted areas near Twenty-nine Palms, CA that is in effect for most of August. Presumably for extended military training.

As the message says, a TRA has the same rules as an RA, just that it is temporary so it won't appear on charts. There may be expected times when the RA is in use but the actual times will be posted in a NOTAM.

It will be published in the Federal Register ahead of time
Using agency notifies controlling agency when airspace will be "hot"
NOTAMs describe when the airspace will be closed
When the airspace is open, ATC can clear flights (IFR or VFR) through

This is useful if the area is known ahead of time but times may be limited and variable. The using agency must also later submit a report detailing the actual usage of the area.
A TFR defines special flight rules that are applicable in specific circumstances. They are governed by different rules, and a NOTAM is published defining the airspace and times, as well as what restrictions are in effect. A TFR may be defined for the following reasons:

Disaster/hazard areas
Protecting the President, the Vice President, or other public figures
Space flight operations
High barometric pressure
Aerial demonstrations and major sporting events
Security requirements

Each of these categories has general rules for who is allowed to operate in the airspace, though each TFR may include more specific requirements. TFRs issued for disasters/hazards generally require aircraft that are not local flights or directly related to the event to be on an IFR flight plan. Other situations may just require permission from ATC, or effectively make the airspace Class C.


text:
Help in decoding this ATC instruction with landing clearance?
I was reading the FAA ATC phraseology document (FAA JO Order 7110.65W) and wanted to see if I could get help on decoding what this phrase meant (from the same document):
“American Two Forty−Five, Runway One−Eight, cleared to land, number two following a United Seven−Thirty−Seven two mile final. Traffic will depart prior to your
arrival.”
Does this mean:
A) American 245 is sequenced to land after the United 737, and the United 737 will vacate the runway before the American lands?
B) American 245 is sequenced to land after the United 737. After the United lands, a different aircraft will use the same runway to depart before the American lands?

B. After the United 737 lands another aircraft will take the runway and depart prior to American 245 landing.
They may not call out what type/make/airline the departing aircraft is. I’ve heard them say ”Company departing prior to your arrival” if the traffic is the same airline though.

FAA JO Order 7110.65W
3−10−6. ANTICIPATING SEPARATION
a. Landing clearance to succeeding aircraft in a landing sequence need not be withheld if you observe the positions of the aircraft and determine that prescribed runway separation will exist when the aircraft crosses the landing threshold. Issue traffic information to the succeeding aircraft if a preceding arrival has not been previously reported and when traffic will be departing prior to their arrival.

B is correct as AL245 is told by ATC to follow and land #2 after the UAL 737 with a different airplane departing the runway before AL245 arrives.


text:
In an ATIS broadcast, is the visibility in statute or nautical miles?
I think I remember seeing somewhere that since it's an observation at the surface, statute miles are used for the visibility. Would be nice if someone had a reference.

In Europe, it's in meters.

...not just in Europe, practically everywhere outside America. Thus, lacking the country-specific tag, metres should be the correct default answer.

The FAA has this handy chart to decode METARs (which are the printed version of the ATIS) and TAFs.

Prevailing visibility; in U.S., Statute Miles & fractions; above 6
  miles in TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as
  required, lowest value with direction)


text:
What's the difference between ATIS and D-ATIS?
What's the difference between the two? Do you need a special digital VHF radio to receive D-ATIS broadcasts?

ATIS is traditionally a voice recording of someone reading the weather. D-ATIS is a way to deliver the information digitally so onboard equipment can interact with it. From the requirements

Digital ATIS is an enhancement of the Tower Data Link Service (TDLS)
  and uses the Pre-Departure Clearance (PDC) System microcomputer to
  automate the delivery of airport and terminal area operational and
  meteorological information to aircraft flightcrews.
  Two phases of development are planned. The first phase will provide,
  via data link, a digital version of the ATIS /4 to flightcrews. Voice
  recording will continue to be utilized in the preparation of the ATIS
  broadcast. The second phase will add automated voice generation for
  the ATIS broadcasts.

Its part of a larger data link system that allows aircrafts and controllers to communicate certain things (weather info, clearances etc) with out clogging the radio channel.
Some ATIS systems now have a voice synthesizer reading out the reports so that no one needs to manually update the recording every hour.


text:
What does "MERMA" mean on the Los Angeles TAC chart?
Just wondering what "MERMA" means on the Los Angeles TAC chart:

How are airspace fixes named?

Should this get more generically-titled somehow, so it might be useful to others?  Perhaps "what does a 5-letter name next to a crossed-arrows symbol mean?"

further out is another one EXERT

MERMA is an aviation waypoint

Waypoints used in aviation are given five-letter names. These names are meant to be pronounceable or have a mnemonic value, so that they may easily be conveyed by voice.

According to this page back before the FAA decided all waypoints must have 5 character names, this waypoint - being out in the ocean - was called "Mermaid" and after renaming this became simply MERMA.

It's an intersection, as described in the FAA Aeronautical Chart User's Guide (VFR Sectional & TAC section):

Named intersections used as reporting points. Arrows are directed
  toward facilities which establish intersection.

If you follow the arrows that make up the MERMA intersection symbol, it's defined by the intersection of radial 246 from the LAX VORTAC and and radial 148 from the FIM VORTAC. MERMA is used in some instrument procedures, like the OCEAN TWO arrival into KLAX.

How can you tell from the chart that it's on radial 148 from the FIM VORTAC?

@200_success If you look on the TAC at the FIM VORTAC you can see V107 follows radial 148 to GINNA and then SADDE; extending that would lead to MERMA. But I admit that's eyeballing it and I had to check the STAR to see if I was correct.


text:
How to fly LOC approach when IAF is a specific DME from LOC, but LOC has no DME?
I was looking at this approach plate and noticed that SNOWL is an initial approach fix, and that it's charted to be 6.4 NM from the localizer I-IWA.
The only problem with starting my approach from the northwest and crossing over SNOWL is that the localizer does not have DME.
First Question:
How can I fly this approach assuming that I'm coming in from the northwest and want to start from the initial approach fix SNOWL?
Second Question:
If SNOWL is 6.4 NM from, why does the profile view show that it's 5.8 (1.9 + 3.9) NM away from the localizer?

You have a few different points here but the short version is: IWA is a VORTAC and I-IWA is a localizer, i.e. they're different navaids. DME is available from IWA and it's optional but not required for the I-IWA localizer approach. If you do have it, you can use it to identify the ORIYE fix and use the lower minimums at the bottom of the plate, e.g. 1800-1 for category A. If you can't identify ORIYE, you can only go to the regular minimums of 1940-1.

First, I suspect you're confusing I-IWA (a localizer) with IWA (a VORTAC) but they're different navaids that use different frequencies, as shown on the chart. The I-IWA localizer provides lateral and vertical (glideslope) guidance but not DME, which you get from the IWA VORTAC. If you don't have a DME receiver, you could use an IFR-certified GPS instead.
Second, if I understood your question correctly about approaching from the northwest, you're asking how to identify SNOWL without using DME. SNOWL is an intersection ("SNOWL INT") and you can identify it using VOR only, by finding the intersection of any two of these radials:

IWA radial 122
PXR radial 109
TFD radial 028

Third, as far as I can see from the plate, DME is the only way to identify ORIYE (remember that you could use GPS instead of an actual DME unit). When you look at the minimums at the bottom of the plate you can see that the regular category A localizer minimums are 1940-1 but there's an extra set of minimums that you can use if you can identify ORIYE and they let you get lower, to 1800-1. In other words, if you have DME then you can use it to identify ORIYE; if you don't, you ignore ORIYE. Either way, you still have to use the correct set of minimums.
Finally, SNOWL is 6.4 DME from the IWA VORTAC, but only 5.8 DME from the "airport profile" (see the chart legend), i.e. the runway. That means, the runway end is 0.6 DME from IWA (that distance is also shown on the profile).

Sweet, that makes sense. Any idea on why it says in the right-bottom portion of the plate that it's 5.8 NM from FAF to MAP? According to the vertical step-down view, that distance should actually be a little less. The MAP should actually be before the airport profile (I'm splitting hairs here, I know). The dashed upward curved line happens before IWA DME 0.6.

the localizer does not have DME.

DME is listed on this chart, so it is available here, though not from the Localizer.  The identifier beside the DME flags show the facility that needs to be tuned to get the required indications.

Are VORs always used instead of localizers on approach plates?

(I assume you're asking about the DME source here)  No.  DME can be located on many different items (VOR, NDB, or ILS).  The chart shows which will be used.  In this case it's the IWA VORTAC.  Some approaches might not have any DME indications at all because there is no DME equipment suitable.
On this chart, waypoints are listed.  The waypoint can be identified through various means.  SNOWL can be identified either through intersecting VOR radials or by the localizer course with DME from the IWL VORTAC.

the profile view shows "IWA" (is that the localizer or the VOR?) with a DME of 0.6

The identifier next to the distance flag shows the equipment that the DME reference is tuned to.  ILS approaches are often paired with DME equipment, so if you have both items available, just tuning the localizer automatically tunes the correct DME.  Normally you don't have to know exactly what the distance is to, you just have to match it.  In other words, if the DME reads "6.4", you shouldn't assume you're 6.4 NM from a runway.

Is the thick horizontal black bar supposed to be the runway?

Basically.  I think the legend defines it as "airport profile".

So if I were to fly this approach, you're saying I should punch in 113.3 into my NAV1 radio, and fly the vertical step-down profile with DME from the 113.3 (IWA VORTAC) station, correct?

It depends on the specifics of your panel.  Some optons:

Tune ILS on NAV1 and VORTAC on NAV2 (and the DME comes in on NAV2)
Tune VORTAC on NAV1, hit "DME HOLD", then tune ILS on NAV1 to bring in the localizer

For an example of a chart where the DME is on the ILS frequency (and you wouldn't have to mess with this), see Las Vegas ILS 26L.  On that one, the DME markings on the profile show the ILS identifier (which starts with I-).

Also note that even though the DME is coming from tuning the localizer, it still doesn't read zero at the threshold.
Unfortunately, I feel I've answered some parts of your question, but not the actual "how to identify SNOWL without DME".  This is a LOC chart, so it should be usable without DME.  I don't see how that part would be done.

Thanks. Are VORs always used instead of localizers on approach plates? And I'm still not understanding the 6.4 NM away from the VOR IWA - the profile view shows "IWA" (is that the localizer or the VOR?) with a DME of 0.6... and it's 0.6 NM away from what? Is the thick horizontal black bar supposed to be the runway?

So if I were to fly this approach, you're saying I should punch in 113.3 into my NAV1 radio, and fly the vertical step-down profile with DME from the 113.3 (IWA VORTAC) station, correct? The profile view shows the DME being referenced from IWA, and not I-IWA.
If that's the case, why is the 110.15 station (I-IWA) listed as the primary NAVAID (top left) of the approach chart?
Sorry for the questions, I just want to make sure I'm understanding it correctly.


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Does the cyclic in a helicopter re-center automatically?
Just wondering if the cyclic in a real helicopter re-centers like a joystick for a computer?
I'm pretty sure the controls in a fixed wing re-centers due to natural physics if the plane is in dynamically stable flight, without the detent (extra force required to overcome fine-grained controls at the center).
Do the controls in a helicopter re-center in the same way? Not sure because I consider helicopters to be fundamentally unstable. I'm also curious about the center detent (if there is one) in a helicopter.

Related

Also related

Helicopters (I have flown so far) does not have such detent positions for their controls just because, as you have said, the instability and the effects of controls on other controls. To reduce the workload some helicopters have systems which probably changes from type to type. While AB-206 helicopters does not have a system for cyclic centering or holding its position, UH-1 helicopters have force-trim system to hold the cyclic where you have centered or left.
AS-532 Cougar ( Eurocopter or Airbus Helicopters now. ) helicopter's cyclic has a cyclic-trim system which returns the cyclic to the position where the pilot set by pushing trim-release button on cyclic.
This is all I can tell for now. Not a complete answer for your question but I hope this helps.


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How do ratings work with pilot certificates?
Just wondering how ratings work with pilot certificates.
Let's say I get both a private pilot certificate and a commercial pilot certificate for airplane / single-engine land.
If I want to add a rotorcraft / helicopter rating onto my commercial pilot certificate for airplane / single-engine land, do I also need to get training to add a rotorcraft / helicopter rating onto my private pilot certificate for airplane / single-engine land also?

Which country are you asking about?

Possible duplicate of What is the difference between a rating and a certificate?

Related: Does the FAA issue multiple physical certificates for different aircraft categories and privileges?

This answer applies to adding a new aircraft category onto your existing Commercial Pilot Certificate in the United States.  Consult the standards of your country's governing body for aeronautics if attempting to do this outside of the USA.
To address the second paragraph, once you hold a Commercial Pilot Certificate for the same category and class of aircraft as your private certificate, you are considered to hold a Commercial Pilot Certificate only.
In regards to the third paragraph, yes additional training will be required to fulfill the requirement of 61.63 (b) for the addition of an aircraft category rating to your CPL.  You will be required to meet the aeronautical experience requirements listed in 61.129 (c) and receive an endorsement in your logbook to take the practical exam in order to add a helicopter category to your CPL.  You will not be required to take the commercial pilot knowledge test again per 61.63 (b).

Can you take the commercial helicopter checkride without first getting private pilot helicopter privileges? Normally, prerequisites for commercial is that one must already have a private.

@Pugz - It's not exactly an answer to your question (category, not class) but I took a commercial seaplane (addon to commercial landplane) checkride without ever taking a private ride.

@SteveV. I have too, but it was only a class add-on, not a category. Hmm.

@Pugz yes, once you meet the Aeronautical experience requirements for the additional category rating and are endorsed by a CFI to do so, you can just take a commercial checkride and bypass a private checkride altogether.

@CarloFelicione sweet. That’s what I thought but I haven’t looked into it.


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How many people can stand on airliner's wings?
This photo of Allegiant Air flight 331 got me wondering: how much weight would a typical airliner's wings be able to hold?

Aside: Apparently these folks didn't realise that if they smell gas in the cabin, they probably shouldn't decide to go out of the emergency exits and stand ON TOP OF ITS TANKS! God forbid one of them gets nervous and lights a smoke!
My question is basically: how much weight are airliner wings required to hold $\div$ the average passenger weight? Theoretically how many people? :)

Just an aside.  You can drop a lit cigarette on jet fuel.  It won't ignite.

@Simon  what about fumes?

No problem in the open air.  I've tried it with Jet-A1 and Avtur.  You have to try really hard to get a jet fuel fire.  You could stand on that wing smoking all day with no problem.  It would be risky during refuelling but otherwise, it;s OK (NB.  I am not suggesting anything - just pointing out that jet fuel is not explosively flammable as suggested by Hollywood and the media). Note to interested readers.  Do not try this with Avpin :)

As many as you like, as long as they mind the NO STEP signs! :)

Shouldn't the wings be able to hold the weight of the entire aircraft? That's what they're designed for after all.

@Ajedi32 Yup. But the air push from the bottom, not from the top... unless you fly inverted of course.

@Antzi Even in inverted flight, the forces remain the same. Airflow relative to the wing does not observe gravity (at least not that it takes any impact on lift)

@SentryRaven if gravity "flips" you need to "flip" also the lift, or you're going to have a close encounter with the ground really fast.

Disregarding the fact that the wing load vector on a passenger jet usually points up, not down, the wings will usually bear most of the load for the entire aircraft. The MTOW for an MD-80 is 63,500 kg. If an "average" passenger is 80 kg, then that is 800 passengers. Obviously that is going to be greater than the number of passengers on the flight, because those wings had already lifted those passengers (and the rest of the plane) into the air in the first place.

Indeed. I'm making some major simplifying assumptions here.

Transport aircraft have to be able to withstand -1G to +2.5G. The -1G is most interesting here, because passengers standing on the wing are causing bending moment in the same direction as if they were in cabin during negative-G manoeuvre (nobody flies airliners inverted, but negative G is sometimes encountered in heavy turbulence).

@JanHudec: In this case you need to subtract the wing's own dry weight twice plus that of fuel inside once, because gravity is pointing the wrong way for the -1g case. But this should leave enough margin.

@TUMBLEWEED Yes, it's always good for the wings to be able to create more than 1G of lift. Otherwise, taking off (or entering a climb, or decreasing descent rate) becomes very problematic. :)

I'm a little confused by this answer. It is my understanding that MTOW is not equivalent to structural load limit, but rather MTOW is the minimum of structural limits and many other factors (e.g. if the wings and engines cannot generate enough lift to fly with some load, that limit becomes the MTOW even if the wings can structurally withstand a much higher load). So it doesn't really seem like MTOW is necessarily an accurate picture of wing load (and especially because the OP's example is for downward loads on the wing, which MTOW doesn't cover - isn't the casually disregarded fact important?)

@Jason C:  You are absolutely correct.  Among those criteria is climb gradient.  The plane must be able to sustain a minimum climb gradient (so many feet per mile) at MGTOW to safely clear all obstacles on its flight path.  And Airport departure routes are also designed and checked to be compliant (so that an engine-out takeoff can in fact fly the route safely if it has to.

The specific numbers will vary by aircraft type and model, but from an empty dry wing, you can add thousands of pounds of fuel to each wing. Then on a winter night, add several pounds of ice/snow to every square foot of the wing's surface, which can easily remain there for hours or days.  Granted, this is all weight that is on the wing rather than on the fuselage, but as was pointed out, these wings support over 50 tons of aircraft weight at cruise and can handle over double that at a sustained G load, so these are pretty stout structures. A couple dozen people just isn't that big a load for a structure that can handle loads WELL over an order of magnitude greater on a fairly routine basis.

Each aircraft has its own design for how much it weight it has to hold on the wing. This is incorporated when an airliner goes through certification (I believe it needs to carry 3 times it's design load for 5 seconds, but I may be wrong in that).
Some airplanes are required to hold lots of weight on the wings, not only from the forces of flight, but also by the possibility of evacuations. According to Wikipedia, aircraft such as the Boeing 747 can hold 740 kg/m2. With a wing area of 525 m2, this means that the wings can hold 388 500 kg. Assuming each passenger is about 100 kilograms, that means you can hold 3885 people on the wing at once. That is, of course, if you can manage to fit them all in that gaggle without slipping off. It'd be pretty tough getting 5 people in a square metre, would it not?

Over 1000 people on EACH WING? Where'd they all come from? Triple economy class seating??? That's actually a good illustration to compare wing capacity with aircraft capacity.

Sorry! Not 1940, but 3885 (How'd you screw that up Zin)! Let just say that we stacked people on top of each other and hope they were skinny noodles, and hoped for the best as well. :)

If I were an airline stockholder, the ability to stuff 2x3885 pax into a cabin, and not being allowed by administrations would haunt me all my life. We need less regulations.

@mins it's only a matter of time..

@mins Well... the plane does have to hold a bit more than just passengers. Itself and the fuel, for example. Passengers are usually actually a relatively small percentage of the overall weight of an airliner, especially in the case of jets designed for long-hauls, like the 747. The fuel alone weighs (a lot) more than the pax. For a 747-400ER, the empty weight plus a full fuel load is 382,334 kg... and the MTOW is only 412,775 kg. In the case of a full fuel load, that leaves only 7.4% of the MTOW for pax+cargo.

@reirab That's true. :)

Everyone pointed out that the total weight won't be a practical limitation. But what may be the limit is the local pressure on the wing skin. Aerodynamic forces are nicely distributed over the wing area. Not so with shoes. A small lady on high heels may do enough damage to ground the aircraft :) Not to mention that some people may start wandering on flaps and other "no step" areas, which may also cause structural damage.

authors: Константин Петрович Кузьмин


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Where does the Departure End of Runway (DER) exactly begin?
I am having trouble understanding the concept of Departure End of Runway. Most sources state that DER is at the end of the portion of the runway that can be used for takeoff (which, as far as I can tell, excludes the threshold at the end of the runway). But from the official FAA's Aeronautical Information Manual one can clearly see that DER starts at the "utmost" end of the runway (including threshold). Many other sources seem to support this, like this picture. Could someone point me to the right direction?

I always understood it to be the „opposite end“ of where you start the takeoff run, i.e. the very first point of the opposite runway... standing by to learn something here!

I'm not sure what context it's being used in, but would expect it does not include the RESA. For more info refer to this question

The picture you linked is from an FAA publication. As pictured the threshold is the end of the runway pavement, it is a thin line that separates usable runway from either taxiway, blast pad, grass, or an emergency overrun area. It is marked at night with a single row of green or red lights (depending on your direction). It can be displaced or reloacated from the original pavment but in this case it will have noticeable markings that include a well defined line marking the exact end. Some ends are usable for takeoff but not landing or vice versa but that is down to individual runways.
The paint bars near the threshold of an instrument runway are simply for visual reference to help indicate the width and length of landing zone during approach, they have nothing to do with takeoff or departure. VFR runways only have a number and dashed center line, no white bars at all.
To summarize: the threshold is a line not an area, it separates usable runway from non-runway, it may be marked with paint or lights but it can also simply be a change in surface such as tall grass to short grass. Standard departure obstacle clearance climb gradients are calculated with the plane at 35feet above the ground, directly over the threshold at the departure end of the runway.

The problem with that picture is that it does not show a displaced threshold. When the white ‘piano keys’ (or the numbers on VFR runways without them) are at the end of the pavement, there is of course no ambiguity. That only occurs when there is a displaced threshold (white arrows).

For departure end, it is the red lights. The green lights visible from the other direction may be at different place in case there is a displaced threshold, no?

According to the FAA's Terminal Instrument Procedures (TERPS), the definition is:

Departure end of runway (DER). The end of the runway opposite the landing threshold (see figure B-1).

ICAO Doc 8168 defines the Departure End of the Runway as "the end of the area declared suitable for take-off (i.e. the end of the runway or clearway as appropriate.)"

Welcome to aviation.SE! Please don't post images of text; it isn't possible to search the text, which makes it more difficult to find answers like this. The [tour] may be helpful if you're new to the site.

Ooops, sorry about that. I thought the screengrab from the official source itself would be good to justify the basis for the answer, especially since ICAO's definition differs from FAA's. In future would it be better to crop the screengrab to just the relevant portion?

Most sources state that DER is at the end of the portion of the runway that can be used for takeoff (which, as far as I can tell, excludes the threshold at the end of the runway).

The displaced threshold, marked with white arrows, can be used for take-off (and roll-out), so the departure end of runway is at the end of that if present.
Blast pad/overrun area, marked with yellow chevrons, and taxiway, marked with yellow edge and possibly centre line, cannot be used for take-off, so the DER is before they begin.

But from the official FAA's Aeronautical Information Manual one can clearly see that DER starts at the "utmost" end of the runway (including threshold). Many other sources seem to support this, like this picture.

The pictures don't show any displaced threshold nor overrun areas, so they are not really indicative of anything.

Just guessing out loud here, but if you go to airnav.com and look up any airfield, two points are given for opposite ends of the runway. For MOR or Morristown, TN,

Runway 5 = Latitude:  36-10.456562N,  Longitude:  083-22.968408W, Elevation:  1312.9 ft. and the other end,
Runway 23 = Latitude: 36-11.069353N, Longitude: 083-22.085700W, Elevation:    1274.4 ft.

As an Engineering Consultant, everything that is done on this runway is done considering these two points - measurements for placing markings, setting up the RSA and all Part 77 surfaces and safety lines, etc. Approach surfaces, departure surfaces, everything is established off these two points, including length of runway, TODA, TORA ASDA, LDA, etc. are all configured off these FAA established runway endpoints, whether there is any extra pavement beyond or not. In my estimation, the DER is going to be the opposite end runway coordinate of the end you are rolling out on.


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Different takeoff minimums between an Obstacle Departure Procedure (ODP) and a Standard Instrument Departure Procedure (SID)
It seems reasonable to me that an Obstacle Departure Procedure would include more "strict" rules (steeper climb gradient) than a SID, which is more "situational" and provides a specific direction in order to enter the en-route structure.
Take, for example, the takeoff minimums for the La Guardia Airport, page L16. It states, for Runway 13, "400-2¼ or std. w/min. climb of 280' per NM to 500".
On the other hand, this SID, also for Runway 13, clearly states as takeoff minimums "Standard with minimum climb of 563' per NM to 710, then ATC climb of 615' per NM to 2500, then ATC climb of 605' per NM to 4000, then ATC climb of 367' per NM to 5000". My question is, why does this SID require steeper climb gradient than the ones listed in Takeoff Minimums for La Guardia Airport?


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Is FAA's Order 8260.58A complementary to the TERPS manual as far as SID design is concerned?
I am studying FAA's United States Standard for Terminal Instrument Procedures (TERPS) to learn how a SID is designed and what kind of rules and constraints one has to take into consideration. I am also studying the PBN Instrument Procedure Design manual (Order 8260.58A), which describes the instrument procedure design for Performance Based Navigation procedures.
However, chapter 14 of the TERPS manual, titled "Departure Procedure Construction", makes no mention of the Order 8260.58 (only once at the beginning for RNAV-equipped aircraft criteria) and proceeds to describe the departure procedures construction as if the Obstacle Clearance Areas that calculates apply to all kinds of departure procedures, without any notice that e.g. "For PBN procedures, consult Order 8260.58".
Could someone help me to clarify how to interpret these two manuals correctly, at least as far as the design of departure procedures is concerned? There seem to be so many rules related to this particular subject and I got a bit lost.


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Is there an equivalent of the Threshold Crossing Height for departure procedures?
The definition for the Threshold Crossing Height is, according to the sources that I have checked, "the theoretical height above the runway threshold at which the aircraft's glideslope antenna would be if the aircraft maintains the trajectory established by the mean ILS glideslope or MLS glidepath" (source). FAA's Aeronautical Information Manual states:

5-4-23 Threshold Crossing Height (TCH) has been traditionally used in “precision” approaches as the height of the glide slope above threshold. With publication of LNAV/VNAV minimums and RNAV descent angles, including graphically depicted descent profiles, TCH also applies to the height of the “descent angle,” or glidepath, at the threshold. Unless otherwise required for larger type aircraft which may be using the IAP, the typical TCH is 30 to 50 feet

Since every runway seems to have its individual pair of TCHs values (one for each Displaced Threshold), I was wondering if there is any similar "Threshold Crossing Height" value that indicates the optimal height above DER (Departure End of the Runway), for each of a runway's thresholds. Does in the case of a departure the "35 feet above DER" rule apply to every runway?

Closely related, perhaps a dupe?

@Pondlife Thank you for your comment, the question you quoted indeed indirectly provides an answer to my own question, since it confirms that the "35 feet above DER" rule applies (with some exceptions) for all runways. The difference is that the questioner takes this rule for granted. I am practically asking if a runway can have custom standards, as with TCH (which I understand now it is not correct), so for this reason I feel that the question is (subtly) different. If you re-post your comment as an answer, I will mark it as the accepted one.